This invention relates to an avalanche photodiode (APD) structure, and more particularly to an APD structure with high multiplication gain and low excess multiplication noise.
Referring to FIG. 1, a typical electron-multiplying SACM (separate absorption, charge, multiplication) APD has a p-doped anode and an n-doped cathode, and between the anode and cathode, in sequence, an absorption layer, a charge layer and a multiplication layer. The APD is used in a reverse bias mode, with the anode of the diode connected to the cathode (negative terminal) of a DC supply and the cathode of the diode connected to the anode (positive terminal) of the DC supply. The material and thickness of the absorption layer (typically InGaAs for near-infrared applications between 1000-1700 nm) is chosen so that there is a relatively high probability that a photon in the desired wavelength range incident upon the APD will generate an electron/hole pair (high quantum efficiency). The macroscopic electric field in an APD junction biased for operation (i.e. the field relevant to the function of the device through its influence on the average motion and energy of charge carriers) causes the hole to drift towards the anode and the electron to drift towards the cathode.
Generally, APDs to be operated in linear (proportional) mode are designed to preferentially avalanche one carrier type, because this condition minimizes the statistical variation of avalanche gain around its mean value (multiplication noise). The choice of preferred carrier type is usually dictated by selection of the multiplication layer material, as contrast between carrier ionization rates is a fundamental material property. For the purpose of the following discussion we shall assume that electrons are the preferred charge carriers unless the context indicates otherwise. It should be understood that the carrier roles can be reversed as dictated by the choice of multiplication layer materials, with attendant reversal of the ordering of absorption, charge, and multiplication layers between the anode and cathode of the APD.
A SACM APD is doped so that the macroscopic electric field in its multiplication layer is higher than in other depleted sections of the APD. Carriers that drift through the multiplication layer are therefore accelerated to relatively high energy levels, and a small population of unusually energetic carriers attain sufficient energy to impact-ionize. Secondary electron/hole pairs created by the collision of energetic carriers with the lattice add to the current flowing in the junction, and are themselves accelerated by the electric field. Secondary holes will drift towards the anode and secondary electrons will drift towards the cathode, possibly impact-ionizing themselves. In this manner, for each primary electron that is photoelectrically generated in the absorber layer, multiple secondary electrons are generated in the multiplication layer and collected at the anode of the DC supply; the average ratio of secondaries to primaries is the multiplication gain of the APD. In the limit of high ionization rates for both carrier types, a positive feedback condition can be established, and the APD will undergo avalanche breakdown. Avalanche breakdown renders the junction conductive, and the multiplication gain essentially goes to infinity.
The propensity of a carrier to impact-ionize depends upon several variables, including its overlap with accessible states that might participate in an ionizing collision, and their density. These factors are jointly constrained by the carrier's energy and the band structure of the semiconductor involved; the existence of a band gap means that below a certain threshold carrier energy, no accessible states will exist for an ionizing collision. Accordingly, different semiconductor materials are characterized by different ionization threshold energies, which roughly track their band gap. Details of band structure and material-dependent transport properties are also responsible for contrast between the carrier ionization rates in the same material.
Since a carrier must accumulate kinetic energy to impact-ionize, and since constant random scattering with phonons and other carriers acts to dissipate accumulated energy, measurable impact-ionization does not turn on until the applied field is so high that carriers have a reasonable probability of accumulating the necessary ionization threshold energy between scattering events. Thus, a relatively high electric field (e.g. 400 kV cm−1 or higher) must be created in the multiplication layer to obtain useable avalanche gain. The field strength required depends upon the material used (e.g. InAlAs) and factors affecting the scattering rate (mean free path), such as lattice temperature and incidence of scattering centers. The material of the absorber layer (typically InGaAs as mentioned above) may have a considerably lower breakdown field than that of the multiplication layer (e.g. about 250 kV cm−1). The thickness and doping of the charge layer are selected to allow the high field to exist in the multiplication layer while a substantially lower field exists in the absorber layer.
The characteristics of the multiplication layer that favor creation of a secondary electron/hole pair from an impact by an electron will generally also favor creation of a secondary electron/hole pair from an impact by a hole, although as noted above, the degree to which each type of collision is favored may contrast.
The signal to noise ratio of an optical receiver based on an APD depends on the excess multiplication noise which in turn depends on the ratio of hole and electron ionization rates in the multiplication layer. Therefore, for a given electron ionization rate, the excess multiplication noise can be reduced by reducing the probability that an impact by a hole will create a secondary electron/hole pair.
The material that is most commonly used as an absorber in an APD designed for the near infrared between 1000-1700 nm is In0.53Ga0.47As and the multiplication layer of such an APD is made of compound semiconductors that are compatible with In0.53Ga0.47As. Unfortunately, such avalanche photodiodes tend to operate with high excess multiplication noise owing to the lack of contrast between electron- and hole-initiated impact ionization rates in the materials involved. Therefore, efforts to construct avalanche photodiodes capable of operating at high multiplication gain (M>>10) with low excess multiplication noise (keff<<0.4) in the near infrared have not been entirely successful.
In the late 1980's and early 1990's, superlattice APDs were developed that relied upon harvesting the potential energy drop associated with carrier propagation over a band edge discontinuity. In certain superlattice material systems such as In0.52Al0.48As/In0.53Ga0.47As, the band edge discontinuity is larger in one band (the conduction band) than the other, so one carrier type (electrons) may tend to receive more of a boost in impact ionization rate than the other. Recent academic criticism has called the mechanism into question, and avalanche photodiodes with very high gain and low keff have not resulted.
In recent years, some researchers have demonstrated suppression of excess multiplication noise by a variety of techniques collectively known as impact-ionization engineering (I2E). In its simplest form, I2E uses the dead space effect to limit the total number of different ionization chains that result from a carrier injected into a thin APD multiplication layer. Dead space is the distance over which a cold carrier must drift under the influence of the multiplying junction's macroscopic electric field before it has picked up sufficient kinetic energy to initiate impact ionization. Dead space may represent a significant fraction of the total volume inside a thin multiplication layer, in which case the spatial localization of impact ionization acts to create a correlation between ionization events that ultimately lowers the multiplication noise of the APD.
A second I2E technique that has been described in the literature is the use of well-and-barrier heterostructure multiplication regions in which the difference in impact ionization threshold energy between two or more materials is exploited to enhance the impact ionization rate of one carrier type over the other. Electrons drift in the opposite direction from holes because they have opposite charge; in a thin heterostructure multiplication region with high threshold material at one end and low threshold material at the other, the ionization rate for the carrier type traveling from high threshold to low threshold will be enhanced, and that of the carrier type traveling from low threshold to high threshold will be suppressed. The reason is that the low threshold material, in which ionization is easy, will be in the dead space of one type, but not the other. A sharper contrast between ionization rates helps to reduce the number of possible ionization chains by eliminating some of those involving feedback, and so the net result is lower excess multiplication noise.
Thus far, it has not been possible to translate the low-multiplication-noise operation of I2E APDs to high gain. This limitation is a consequence of the requirement that their multiplication layers be very thin in order to benefit from the dead-space effect. A thin multiplication layer has low noise because the number of possible ionization chains is small; by the same token, the long ionization chains necessary to get high gain cannot fit inside a thin multiplication layer. Higher gain can be eked out of a thin multiplication layer by increasing the field strength, but in doing so, the contrast between ionization rates is lost, feedback is enhanced, and noise suppression is lost precisely because a larger number of ionization chains can now fit into the same space. The stronger fields not only degrade excess noise performance but also enhance dark current leakage mechanisms such as band-to-band tunneling and thermionic field emission.
It might at first appear that if individual I2E multiplication layers cannot be operated at high gain and still preserve their low noise character, it would be possible to achieve high gain by employing several multiplication layers each operating at low gain and cascading them in stages. However, simply growing a series of I2E multiplication layers is not sufficient: without some way to prevent feedback between stages, all that is obtained by stacking a series of thin multiplication layers is a single thick (and noisy) multiplication layer.
U.S. Pat. No. 6,747,296 B1 describes an avalanche photodiode with a cascaded multiplication structure, with each stage of the multiplication structure being composed of multiple layers. Each multiplying stage comprises (in the direction of travel of electrons, which are the preferred charge carriers) a layer of a first material M1 and a layer of a second material M2, the impact ionization threshold of the second material being lower than that of the first material. The layer of the first material includes (in succession, in the direction of travel of electrons) a first intrinsic region, a p-doped region, a second intrinsic region, and an n-doped region. The layer of the second material is intrinsic. Thus, the placement of doping is such as to raise the macroscopic electric field strength to its maximum in the layer of material having the higher ionization threshold, and to lower it within that layer. This multiplication structure is intended to result in the probability of an ionizing impact being a maximum when an electron enters the material M2, where the field is lower than the maximum field in the layer of the material M1.
U.S. Pat. No. 6,747,296 B1 describes a hole step-down region in connection with FIGS. 3A-3D. The function of the hole step-down region is described in terms of band edge discontinuities. Thus, various forms of grading in composition are described as either (a) preventing non-preferred charge carriers from harvesting band edge discontinuity energy to enhance their impact ionization rate, or (b) facilitating transport of preferred charge carriers across the band discontinuity in order to increase device speed. For this reason, U.S. Pat. No. 6,747,296 B1 discloses that the material used in the hole step-down region must be of intermediate band gap. U.S. Pat. No. 6,747,296 B1 does not prescribe any requirements regarding the relative electric field strength in the hole step-down regions.